Show scrollbar when necessary (go-gitea#20142)

- Backport go-gitea#20142
  - Firefox on Windows will unconditionally show scrollbars when you specify `overflow: scroll`. This is bad behavior, as you don't always need the scrollbar. Changing the scroll value to auto fixes this issue and only shows the scrollbar when necessary.
  - Resolves go-gitea#20139

web_src/less/_repository.less

@@ -3051,7 +3051,7 @@ td.blob-excerpt {
   align-items: center;
   display: flex;
   justify-content: space-between;
-  overflow-x: scroll;
+  overflow-x: auto;
   padding: 8px 12px !important;
 }